1. Field of the Invention
The present invention generally relates to image display apparatuses, signal processing apparatuses, image processing methods, and computer program products. More particularly, the invention relates to an image display apparatus that can reduce the occurrence of blurring phenomenon by performing interlace-to-progressive (IP) conversion for converting interlace signals into progressive signals when displaying images on a frame-hold-type display, such as a liquid crystal display (LCD). The invention also relates to a signal processing apparatus, an image processing method, and a computer program product used in the image display apparatus.
2. Description of the Related Art
In display processing utilizing flat panel displays (FPDS) using organic electroluminescence (EL) or liquid crystals (LCs), frame-hold-type display is performed, unlike cathode ray tubes (CRTs) employing dot-sequential impulse driving display. That is, in a typical FPD operating, for example, at a frame frequency of 60 Hz, during every display period ( 1/60 sec=16.7 msec) of one frame, the same image is continuously displayed (held) on the whole display screen.
In such frame-hold-type display, image blurring occurs due to afterimage remaining on the retina. More specifically, when displaying a moving object on a frame-hold-type display, such as an FPD, the image picked up by the retina appears to jump while the eye is following the displayed moving object, which makes the moving object appear blurred. Because of this blurring, the quality of moving pictures is deteriorated.
As one measure to reduce the occurrence of blurring phenomenon, a so-called “black insertion” technique has been proposed. In this black insertion technique, a high-speed-response display device operating, for example, at a frame frequency of 120 Hz, is employed, and an actual display image is first displayed in a period of 1/120 sec, and a black color is displayed in the next 1/120-sec period, and then, another actual display image is displayed in the next 1/120-sec period, and then, a black color is displayed in the next 1/120-sec period. That is, by the insertion of a black color between frames to be displayed, the FPD is allowed to perform pseudo-impulse-driving operation. By simply inserting a black color frame, however, the brightness of the display image including the black color is integrated on the retina of a viewer, which reduces the brightness or contrast level of the display image.
To solve this problem, for example, the following configuration has been proposed in Japanese Patent Unexamined Application Publication No. 2005-128488. In this configuration, the rate is increased by n times (xn), and then, a video signal having a luminance level lower than that of the original frame is inserted as a sub-frame, so that a trade-off relationship between the impulse driving and the brightness or contrast can be implemented.
Japanese Patent Unexamined Application Publication No. 2005-173387 discloses another configuration. In this configuration, a video signal in the period of one frame is divided into a plurality of sub-frames in a time division manner, and then, the allocation of luminance components among the divided sub-frames is adjusted so that the integrated luminance obtained by integrating the luminance components of the divided sub-frames is comparable to the luminance of the original frame. As a result, pseudo-impulse driving can be implemented without impairing the brightness level.
In the configuration disclosed in Japanese Patent Unexamined Application Publication No. 2005-128488, however, there is a tradeoff relationship between impulse driving and the brightness or contrast, and it is difficult to avoid a decrease in the brightness or contrast to a certain extent. In the configuration disclosed in Japanese Patent Unexamined Application Publication No. 2005-173387, even if a suitable allocation of luminance components among the time-divided sub-frames is performed, a sufficient effect may not be obtained, depending on the luminance level of the pixels of the original frame. Additionally, it is necessary to set time-divided frames having pixel values with luminance levels lower than the luminance levels of the pixel values forming the original image, in which case, if the luminance levels of the pixels of the original frames are low, it is difficult to set time-divided frames having suitable pixel values.
Currently, most of the content pieces or broadcast signals used for displaying images are generated as image data in accordance with the CRT-compatible interlace driving. More specifically, one image to be displayed in the horizontal scanning lines of a CRT display is divided into two fields, and in one field, every other horizontal scanning line is scanned from the top to the bottom, and then, in the other field, the remaining horizontal scanning lines that have not been scanned are scanned from the top to the bottom, so that the entire image can be displayed. There are many interlace image content pieces that are generated as discussed above. That is, repeatedly scanning every other horizontal scanning line from the top to the bottom generates one image.
If such interlace image content is displayed on a frame-hold-type display device, lines associated with display image signals and lines not being associated with display image signals are alternately generated, and flicker becomes noticeable, and also, the luminance level is reduced. To solve this problem, an interlace signal is converted into a progressive signal, and then, the image is displayed. As stated above, processing for converting an interlace signal into a progressive signal is referred to as “IP conversion”.
Generally, in a CRT display, scanning every other horizontal scanning line from the top to the bottom of a screen is referred to as “interlace scanning”, while sequentially scanning a plurality of horizontal scanning lines (horizontal display lines) forming the screen line by line is referred to as “progressive scanning” (sequential scanning). In the progressive scanning, pixel signals corresponding to all the scanning lines are provided.
In IP conversion for converting interlace signals into progressive signals, lines not being associated with signals contained in the interlace signals are generated by interpolation processing. By the application of pseudo-signals generated by this interpolation processing, interlace signals can be converted into progressive signals including information on all the pixels.
The interpolation processing used in the IP conversion is performed on the basis of the pixel values of surrounding pixels adjacent to existing pixels in the spatial or temporal direction. In many cases, pixel values similar to those of surrounding pixels are set for pixels to be interpolated. This accelerates the above-described blurring phenomenon.
In the IP conversion for converting interlace signals into progressive signals, the pixel values of pseudo-pixels are estimated and determined on the basis of the pixels values of the surrounding pixels in the spatial or temporal direction. Accordingly, users have to view content partially replaced by pseudo-pixel values, which is annoying for users who desire the faithful playback of original content.